This invention relates to an air-fuel ratio controller of an engine.
A catalyst which purifies engine exhaust known in the art which has oxygen storage capacity, absorbs oxygen when the air-fuel ratio of the exhaust is lean, and releases the absorbed oxygen when the air-fuel ratio of the exhaust is rich. This is disclosed in JP 5-195842A and JP 7-259602A which are Japanese Patent Publications.
Therefore, in this catalyst, when the air-fuel ratio of the exhaust varies slightly from stoichiometric to rich or lean, the catalyst atmosphere can be maintained at stoichiometric so that oxidation of HC, CO and reduction of NOx are both performed well.
However, there is a limit to the oxygen storage amount of the catalyst. If this is exceeded, the catalyst atmosphere becomes lean, and moreover, when the air-fuel ratio is rich and the oxygen storage amount becomes zero, the catalyst atmosphere becomes rich.
As a result, if the air-fuel ratio is controlled so that the oxygen storage amount of the catalyst is always about xc2xd of the maximum oxygen storage amount, the oxygen absorption and release capacities of the catalyst are equalized so that it is possible to cope when the air-fuel ratio fluctuates to either rich or lean from stoichiometric.
For this purpose, it determines whether the oxygen in the exhaust flowing into the catalyst is insufficient or excessive based on the detection value of this air-fuel ratio sensor placed upstream of the catalyst, estimates the oxygen amount stored in the catalyst, and controls an air-fuel ratio so that this storage amount is a target value.
However, as the air-fuel ratio sensor installed upstream of the catalyst comes in direct contact with high temperature exhaust, its performance deteriorates due to the effect of the hot exhaust, and errors may appear in the detection of the air-fuel ratio. In this case, the output of the air-fuel ratio sensor shifts relatively to either rich or lean. This may also occur due to scatter in the quality of the air-fuel ratio sensor when it is manufactured.
If there are errors in the detected air-fuel ratio, the computation of the oxygen storage amount in the catalyst which is based on the output of the air-fuel ratio sensor may be incorrect, and it may be difficult to precisely control the oxygen storage amount of the catalyst to the target value. In this case, the exhaust purification efficiency of the catalyst decreases.
It is therefore an object of this invention to correctly determine whether or not there are errors in the output of an air-fuel ratio sensor.
It is a further object of this invention to correct output errors when such errors occur in the output of the air-fuel ratio sensor, and precisely control the oxygen storage amount to the target value.
In order to achieve the above object, the invention provides an engine air-fuel ratio controller which comprises a catalyst installed in an exhaust passage which absorbs oxygen when an exhaust air-fuel ratio is lean, and releases the absorbed oxygen when the exhaust air-fuel ratio is rich, an air-fuel ratio sensor installed upstream of the catalyst, which detects an air-fuel ratio upstream of the catalyst, an air-fuel ratio sensor installed downstream of the catalyst, which detects an air-fuel ratio downstream of the catalyst, and a microprocessor.
The microprocessor is programmed to control a fuel supply amount of the engine to obtain the stoichiometric air-fuel ratio, which is a target air-fuel ratio, based on the detection value of the upstream air-fuel ratio sensor, to estimate the oxygen storage amount absorbed by the catalyst based on the detection value of the upstream air-fuel ratio sensor, to modify the target air-fuel ratio so that the estimated oxygen storage amount coincides with the target value, and to determine whether or not there is an error in the output of the upstream air-fuel ratio sensor based on the detection value of the downstream air-fuel ratio sensor, and correct the detection value of the upstream air-fuel sensor according to this determination result, and determine that there is a fault in the upstream air-fuel ratio sensor when the absolute value of the integral of the detection values of the upstream air-fuel ratio sensor exceeds a predetermined value.
The details as well as other features and advantages of the invention are set forth in the remainder of the specification and are shown in the accompanying drawings.